Electricity & Magnetism Flashcards
Define Electricity
A form of energy associated with stationary or moving electrons or other charged particles
Electric charges exert …
forces on one another
Like charges…
repel
Opposite charges…
attract
The magnitude if the force between the charges depends upon…
the medium in which they lie and its maximum in a vacuum
The magnitude of force is inversely proportional to…
the distance between the charges, the smaller the distance, the greater the force
Coulomb’s Law
F is inversely proportional to q1xq2 / d2
F = force (newtons)
q1 and q2 = 2 charges (coulombs)
d = distance apart (metres)
Describe a photon and give an example
a small bundle of energy with no charge and no mass, x-rays and gamma rays are photons
Describe charge
measured in coulombs, a single charge can exist in isolation, 1 coulomb is the charge carried by 6.25 x 10 to the power 18 electrons
1 coulomb is …
the amount of charge that flows during 1 second if there is a current of 1 ampere
Define Field and give an example
A field is any region in space where a particular entity exerts its influence - e.g. electric, magnetic, gravitational
(Always remember fields are 3D)
The direction the lines point in an electric charge diagram show …
the direction in which an isolated POSITIVE charge would move if placed at any point in the field
The strength of a field is indicated by…
how close the lines are together
When an electric field is associated with 2 parallel plates, the direction of the electric field is …
postive to negative
A postive electrode is called …
anode
A negative electrode is called …
cathode
When does an electron move
when there is a difference in the electrical potential or ‘potential difference’ (p.d.)
What is the unit of p.d.
volts (V)
Why can we measure energy in electronvolts (eV)
because an electron moves when placed in an electric field - as it moves it gains kinetic energy, the higher the voltage, the more kinetic energy it gains
Describe the electronvolt (eV)
This is the amount of energy gained by an electron when it moves through a p.d. of 1 volt
Describe an electric current
whenever a charged particle moves (A)
Material through which a current flows is called a …
conductor
Electric currents can flow through
solid, gases and vacuums
To have a circuits there must be …
a potential difference and no gaps
How are Voltage, Current and Resistance in a circuit related?
By Ohm’s Law
Describe Ohm’s Law
The current (I) flowing through a metallic conductor is proportional to the potential difference (V) across it, provided that all its physical conditions (e.g. its temperature) remains constant I is proportional V
Describe resistance
the opposition a material presents to a flow of electrons - measured in ohms
Describe an insulator
a material whose electrical resistance is so high that current cannot normally flow through it (e.g. rubber, plastic, wood)
The resistance of a conductor can be introduced to Ohm’s Law and rewritten …
I = V/R
Definition of the Ohm
A conductor is said to have an electrical resistance of 1 ohm if a potential difference of 1 volt across it produces an electrical current of 1 ampere through it
Transformers
increase or decrease voltage
There are 2 types of magnetic pole …
north & south, like poles = repel, opposite poles = attract
Main/Only difference between electricity and magnetism
A single magnetic pole cannot exist in isolation but a single charge can exist in isolation
Magnetic Flux Density
the number of lines in a given area of space, measured in Tesla
A magnetic field is only present when …
the current is on
Formula to find the force between two magnetic poles
F is proportional to m1 x m2 / d2
The lines of force are …
circular and concentric with the long axis of the wire
The direction of the field depends on ..
the direction of the current
Name the 3 factors affecting the strength of the magnetic field in a current-carrying solenoid
The number of turns - as number increases, the number of lines of magnetic flux increase in proportion
Current - as it increases through the coiled wire, the magnetic field strength increases up to a maximum
presence of ferromagnetic (soft iron) core - inserting a care through the centre of the wire greatly increases the magnetic field
If a current is flowing through a resistance (e.g. wire) it causes …
heat to be generated and therefore wastes energy
Most MRI systems use … explain …
a superconducting magnet - consists of many coils of wire through which an electric current is passed creating a magnetic field of up to 2.0 Tesla
Maintaining such a large magnetic field consumes energy, explain how this is minimised …
minimised by reducing the resistance in the wire to almost zero by continually bathing them in liquid helium 269.1 below zero Celsius - insulated by a vacuum
A strong magnetic field allows for … and superconductivity allows for …
high quality images, the system to operate economically
